1. Field of the Invention
The present invention relates generally to a fiber gratings (or a fiber Bragg grating) as an optical fiber device, and in particular, to an apparatus and method for fabricating a fiber grating.
2. Description of the Related Art
Fiber gratings are written in an optical fiber by irradiating the core of the optical fiber with laser beams emitted from a UV (UltraViolet) excimer laser (e.g., a KrF or ArF excimer laser) in order to incur periodic variations in the refractive index of the core. The refractive index of the core increases as it is exposed longer to the laser beams and the periodic refractive index variation is effected by projecting the laser beams along the length direction of the core periodically.
The fiber gratings selectively reflect a specific wavelength component of light traveling along the core or leak the wavelength component into a clad according to a refractive index variation period, refractive variation magnitude, and grating length. The reflected or leaked wavelength component is termed a central wavelength.
Research has been actively conducted on development of an optical fiber laser using fiber gratings and application of a fiber grating to an optical fiber sensor and many modifications or improvements have been made to the fiber gratings, such as a chirped fiber grating, a long period grating, and a tilted grating. Recently, fiber gratings have widely been used for dispersion compensation of an optical fiber, pulse compression, sensing, and flattening of EDF (Erbium Doped Fiber) gain spectrum.
Fiber gratings are fabricated by a holographic method, a phase mask method, or a point by point method.
The holographic method is called an interference method, in which periodic patterns are written in the core of an optical fiber according to interference spectrum by splitting a laser beam through a beam splitter and combining the split beams through reflective mirrors. The holographic method has the distinctive shortcomings of difficult system alignment and requirement of an expensive laser with a great coherence distance.
A phase mask is a diffractive optical element (DOE) that serves to branch an incident laser beam in diffraction directions of different orders. To fabricate fiber gratings using such a phase mask, a zero-order diffraction pattern of a laser beam perpendicularly incident on the phase mask is suppressed to several % or below and the other laser beams are diffracted in first-order directions by setting the height of gratings to an appropriate level in the phase mask. Thus, the diffracted light incur interference in the core. As compared to the holographic method, the phase mask using method can be implemented with a cheap industrial laser due to its advantages of system simplicity, stable formation of gratings, and little influence from temporal coherence characteristics of a laser beam.
The formation of fiber gratings is discussed in the following references incorporated by reference herein: U.S. Pat. No. 5,620,495 to Jennifer Aspell et al. entitled Formation Of Gratings In Polymer-Coated Optical Fibers; U.S. Pat. No. 5,818,988 to Robert A Modavis entitled Method Of Forming A Grating In An Optical Waveguide; U.S. Pat. No. 5,830,622 to John Canning et al. entitled Optical Grating; and U.S. Pat. No. 6,072,926 to Martin Cole et al. entitled Optical Waveguide Grating.
A conventional fiber grating fabricating apparatus includes a high power laser source for emitting a UV laser beam, a lens unit for focusing the UV laser beam, a phase mask through which the focused light is projected, and a fixture closely disposed to the phase mask for fixing an optical fiber in which gratings will be written. A reflective mirror may be further included to help the light emitted from the laser source accurately impinge on the lens unit by changing the path of the light. Prior to fixing the optical fiber in the fixture, a coating is peeled off from grating forming portions of the optical fiber. The phase mask is arranged in contact with the coating-removed portions of the optical fiber. A UV laser beam emitted from the laser source is focused on the phase mask through the lens unit. The laser beam incurs interference in the photosensitive core of the optical fiber through the phase mask and then gratings are written in the core in an appropriate period. The gratings are completed by forming a coating on the gratings-formed portions of the optical fiber.
Technology of narrowing channel spacing or widening an available wavelength band is under development in the field of WDM (Wavelength Division Multiplexing) transmission that is a widely employed scheme for an optical transmission system. This situation requires that a fiber grating used for an optical fiber laser or filter should operate more accurately.
With high accuracy, therefore, the laser source, the lens unit, and the phase mask should be positioned and accurately on the fiber to form the grating. Accordingly, only one fiber having a grating formed therein can be made. As a result, it takes a long time to fabricate one fiber grating on a single fiber, thus the product yield is low and cost is high.
An object of the present invention is, therefore, to provide an apparatus and method for fabricating a fiber grating with a higher product yield.
Another object of the present invention is to provide an apparatus and method for fabricating a fiber grating to be used as a sensor with a higher product yield.
To achieve the above objects, there is provided an apparatus and method for fabricating fiber gratings. To fabricate fiber gratings using a mask having gratings written therein, at least two optical fibers are arranged in parallel, in which the fiber gratings will be written by periodic variations in the refractive indexes of the photosensitive cores thereof, the mask is disposed on the optical fibers to cover gratings-forming portions of the optical fibers, and a laser beam is projected on the mask in a focused light size covering the gratings-forming portions of the optical fibers, to change the refractive indexes of the cores.